Automatic spectrophotometer



Aug. 1, 1950 M. D. LlsToN AUTOMATIC SPECTROPHOTOMETER Filed June 5, 1947 www Q J gw, |.l gi/f l, a ,w

f l attorney Patented Aug. 1, 1950 AUTOMATIC SPECTROPHOTOMETER Max D. Liston, Wilton, Conn., assignor to General Motors Corporation, Detroit, Mich., a corpora;-

tion of Delaware Application June 3, 1947, Serial'No. 752,120

(Cl. Z50-43) 13 Claims.

My' invention relates to improvements in means for measuring the relative radiant energy from a source as a function oi the wave length, or what are known as spectrophotometers ici` quantitative analysis Work.

While spectrophotometry is in general a wellknown art, the means utilized heretofore have had many limitations, one of which has been, that, since the amount of energy which reaches the detecting device is exceedingly small, a very sensitive galvanometer was used therewith, which was actuated by the output of the thermopile or other sensitive pick-up means. The currents available being very small, the sensitive galva nometers indicating the same were adversely aifected by mechanical vibration in the vicinity of the measurements. A satisfactory direct current amplifier has, however, been recently developed, and is used in my general system. This amplifier, while itself forming no part of my invention, in combination with othe1` elements forms novel means. This D. C. amplifier is disclosed in a patent issued to Sargeant and Hoeper #2,413,788, issued January 7, 1947, The use of this amplier so increases the power of the currents involved that a recording meter may now oe used in the place of the sensitive galvanometer.

In equipment of this type it is a very great aid to produce some permanent record of the results of the measurements and to have this record visible as it is made and immediately available to the operator upon its completion. This was, oi course, not practical with the galvan'ometer, but the recorder now provides such a permanent and immediately available record. A D. C. sy tem of this type provides great sensitivity and high speed response to'signal. It has, however, one disadvantage, and that is that it is subject to drift, but this is minimized in the present case by certain features to be described.

In a spectrophotometer, maximum resolution is 'necessary to provide accurate measurements. This means that a portion of the spectrum falling on the sensitive receiver at any one time must include as short a wave length range as possible so that features of the spectrum which are close together in terms of wave length may be shown separately on the record and not be confused by overlapping. Resolution is primarily controlled by varying the Width of the slits in the spectrophotometer, but this slit width also controls the energy passed through the instrument to the detecting means and this cannot be made too small lest the indications of features of the spectrum' become less than the existing random disturbance, or noise in the system. Thus the slit adjustment isat allzpoints in the spectrum a compromise between resolution desired and energy available.

An object of my invention is to provide an automatic spectrophotometer that provides a perman'ent and immediately available record of the results obtained by scanning the spectrum.

It is a further object of my invention to provide a spectrophotometer that maintains maximum resolution in all parts' of the spectrum.

ItV is a stili further object of my invention to provid-e` ay spectrophotometer that automatically adjusts the energy passage controlling means as the spectrum is scanned.

It is a still further object of my invention to provide a spectrophotometer system which is a null system, and balances at different timed steps and is automatically corrected for zero flow at these stages.

It is a still' further' object of my invention to vary the energy entering the spectrophotometer periodically during the recording of a spectrum toc'ompensate for changes in energy emitted by the source in different parts of the spectrum.

It is a still further object of my invention to provide means to adjust controlling slits periodically during the recording of the spectrum so that in eachA part the resolution obtained is as large as consistent with the energy available from the source in that part of the spectrum, the adjustment means being automatic and in accordance with a preselected program.

It is a still further object of. my invention to provide means to preselect the points in the spectrum where the recording will begin and end and the automatic periodic adjustment of slits between these limits being accomplished by remoteelectrical means Without any direct mechanical connection.

Withv these and other objectsin view which will become apparent as the specication proceeds, my invention willbe best understood by reference to the following specification and claims and the illustrations in the accompanying drawings, in which:

The gure shows a schematic wiring diagram of a system embodying theY details of my invention.

InV this iigure the source of radiant energy 2, which provides thebeam of rays which are to be projected through the sample 4, may be oi any sort of relatively brilliant constant illumination. In the present case, vthere is used what is known as a Nernst glower, which projects a beam through said sample 4, which may be of any material which it is desired to examine and test, through an adjustable slit 6, and onto one side of the prism 8, the prism, of course, refracting and breaking up the beam into a spectrum. These rays then proceed through a second adjustable slit l0, and impinge upon thev pick-up means which is a thermopile |`2. The two adjustable slits 6 and 0 are connected to be simultaneously adjusted as shown by dotted lines, and thence through a further dotted line indicated as |4 down to a small motor I6, so that the widths of these slits may be varied, depending upon the circumstances of the test. The motor |6 is provided to drive the change width mechanism for these slits as the occasion arises, and-is controlled by a system to be described.

Between the source 2 and the sample 4, there is provided a shutter I 8, which is operated by rack 2Q, and a solenoid coil 22, which coil is connected through line 24 back to an electronic timer control 26, which contains anumber of program sequence timed switches whose specific construction is not necessary to the understanding of my invention, but which, when operated in this instance, causes the shutter I8 to flip up and intercept the rays from the source 2 for various adjusting purposes. 1 Y

The output of the thermopile I2 flows through line .28 to a mechanical contact modulated amplifier -30 of the type disclosedin the Sargeant and Hoeper patent above referred to. The opposite side of the thermopile is, in like manner, connected through line 32, and a plurality of resistances 34, 36 and 38 in: series, and thence to the other terminal of the input of the amplifier. A line 48, extending from the junction of the resistors 36 and 38 extends up to one pole of a small battery 42, the oppositefpole of which is connected to an adjustable switch contact 44, which may be placed in contact with any one of a 'series of parallel resistors 46, 48, 50 and 52, all of which have their opposite terminals connected to a line 54, which joins the lconnection between the resistor 38 and the 'amplier 3|l. Line 40 also extends to a small battery 56, which is in turn connected to a resistor 58, the opposite terminal of which is connected to an adjustable tap 68 on resistance 62, which is connected` directly in shunt across resistances 36 and 38. The adjustable contact 44,"whic`h cooperates with the various resistances 46 through v52, is for the purpose of applying'test signals to the amplier in order to adjust the's'ame. 'The' purpose of the adjustable tap 60, which cooperates with the resistance 62, is to.` balance the bridge or zero the'system 'at each variation or re-setting of the device, and it' is'driven'by a zeroing motor 64, as shown by the dotted line connection between the two. y

In the output of the amplifier 38, there are connected means for controlling the speed of response of the signal to the recorder, and the-se involve, in one of the output lines, 'a pair of resistanceslt` and 68 in series, the most lremote contact of the last resistance 68 being connected to a conductive control line 10. The opposite terminal of the output of the ampliliel` 30 is connected to line l2, which is grounded as shown. A series of condensers 14,' 16, 18 and 80 have one terminal connected to this line, and the opposite terminal connected to a stationary contact which is adapted to cooperate with a movable tap contact member 82. 'This is in turn GQnnSCiEKl C0 3i fi point between the two resistances 66 and 68, and also to one terminal of a resistor 84. Contact 82 may, therefore, be moved back and forth to include various amounts of capacity in the system. Resistor 84 is engaged by an adjustable tap 85, which is connected in turn to line 88, which is connected to a stationary contact 90, cooperating With an adjustable contact 92, the latter being moved by a'mechanical rod 94, whose movement is again controlled by the electronic timer 26. This is accomplished by a small solenoid coil 96, whose energization is controlled by said timer. The lines lil and 'l2 forming the output of the amplifier 38 are connected to two lines 98 and |88, which are the input to a recorder amplier |82, which together with other elements is the f rugged recorder referred to previously and which produces the visible and permanent record of the results ofthe test. This recorder includes a balancing motor |54, which operates the recording per, |65, as shown by the dotted line, said pen engaging a continuously driven paper |83, providing a. chart or graph. A drive for the paper chart is indicated as lll), and receives its power from line H2 and the electronicftimer control. The motor |94 also drives a small generator l i4, which may be termed the anti-hunt generator, and to one of whose output terminals is connected resistance l I6, in series with a resistance I i8, the latter being a-Variable resistance, and having a tap |28 thereon, which is connected to line |22 extending to a junction with line lil. The opposite side of the generator is connected through line |24 directly to line l2. A condenser |26 is connected from line |24 to the-junction point between the two resistances H6 and H8.

Motor |84 also drives one additional member, which is the adjustable balance tap |28 for the recorder, which engages a resistance means |323, which is connected in series with battery |32, and a second resistance means |34; A ground is connected between resistor |36 and the battery |32. This latter also has an adjustable tap |35, which. is directly connected back to complete the series circuitwith the opposite end oi resistance |36.

As the tap |28 moves along the surface of the resistance or slide wire |36, the recorder is balanced. The movement of the tap |36 along its resistance |34 is an initial o-r coarse adjustment of gain. Adjustable tap |28 is connected through line |38 with a second adjustable tap |46, contacting resistor or slide wire |42 for so-called fine gain adjustment, whereas |36 and |34 variation was for coarse gain. Resistance |42 is connected through line |44 to the movable contact 92 previously mentioned.

The motor |84 is of a reversible type, and is capable of rotation in either direction, being controlled by switches MES-|48, which are included between lines |58 and |52. The movable switch arm |46 is rigidly secured to the rod 84, s-o that this switch is actuated simultaneously with movable switch arm 92. In the opposite positions, switch arm |46 engages contact |54 and thus controls the energization of the zeroing motor 61|, previously described; Motor |64 is controlled in addition by the electronic timer control through line |55. One other switch is actuated by movement of the arm 94, and that is the movable switch arm |58, which engages stationary contacts |66 and |52. This movable arm is connected to line |64, which extends to line 18. Stationary contact |60 is connected through line |66 to adjustable tap |68, engaging resistor iid, and contact |62 is connected through line H2 to adjustable tap l'l4, slidably engaging resistor 1:15 the latter being directly connected to the opposite end of resistor l'l and to line T2. Slidable contact V'fl is mechanically ganged' to move with slidable contact |36, asshown by the dotted' line connection between the two. The sliding. contact lili is provided to adjust the dead. spot control :for the recorder, and adjustable contact l 68 is'provided for the'so-called zeroingdead spot control, duringy the zeroing operation.

In order t scan through the spectrum the prism 8 is placed so that one end of said spectrum is focused through the slit i0 onto the'thermopile, and then the mechanism is set in operation, the prism being driven or turned very slowly so that the full gamut of the spectrum will be allowed. to pass through. the slit l0 as the prism rotates. The prism drive is provided by motor W8, as shown by the dotted line connectionl to the prism, and this motor simultaneously turns a number of other devices, the first being a com mutator i893 for switching', the seconda a pulse generator 82 for providing time signal pulses on the record and also a triggering pulse for actuation of the system which will be explained more in detail later, the third and last being a commutator switching means 9.84 for prism limit control, `which prevents the same from going beyond the pre-determined limit. rThis last means is connected by line itt with the prism motor control i558, the other control means for this member being the electronic timer control which exerts its inuence through line iil to the prism motor control E88, the output Alovving through line 92 to the motor per se.

The movable or sweeping portion 184 of the commutator slit control is connected through line St to a stationary contact l9il of a switch', the setting of which determines whether the operation of the system shall' be automatic or manual. As the member 94 sweeps around the various 'contacts .288, it brings into lthe circuit various adjustable potentiometers 262, 204, 286, etc., through various conducting lines 208, 210, 222, etc. These adjustable potentiometers hav ing taps 2id, Zit` or 218, which may be moved up and down, can be pre-set so that when brought into the circuit they will cause the' slit motor to balance at different pre-determined mechanical positions, depending upon their setting. One terminal of each of the resistors 2e'2, '204, 26.6', etc., is connected to line 22S, which terminates in an adjustable contactor 222 which engages4 a series of iiXed contacts between a plurality of series resistors 224-, 226, 228, and 23d. The rst of these resistors 221i is connected to a line 232. These series resistors form a step resolution control for the whole system. The opposite terminals of the various potentiometerssare connected to line 2534. Connected across between lines 232 and .234 in parallel relation are a pair oiresistors 226 and 238 and a battery 240. A variable tap engaging resistor 235i is connected through line 241i to stationary contact 245, and an: adjustable tap 243, which engages resistance 238 in a similar manner, is connected directly intol a modulated D. C. amplifier 25B. These resistors form a balanced bridge for the amplifier input when the system is being operated manually without the automatic program means for changing the slit widths for resolution. When the automatic program means is in circuit the balancing bridge circuit is formed of resistance 238-and one of the resistances 202, 284. A manually movable switch Contact 252 can. be kmoved backv and forth y.to-en.-

gage: either contact |9801' 246, and is connected through line 254 to the input of the amplifier 258'. This switch determines whether the device is operating on a. iixed'. manual adjustment or the automatic program.v

The output of the amplifier 25D has one of its terminals connected through lines 255 and 258 to one of the windings 260 of the motor I6, the opposite terminal of the winding being connected through line 262 to two movable switch arms 264 and .226; the first of these switches 284 being a manually operated switch normally open and the second 266 being an electricalsolenoid switch which is closed upon signal from the electronic timer, being operated by solenoid 258, The stationaryv contacts 2li! and 212 which cooperate with switches 264 and 266 respectively are connected directly to line 224, which goes to the amplifier 25D. Line 258 also extends to a solenoid actuating coil 216, the opposite terminal of which is 'connected to a resistor 2*'58 and back to liney 21.4, so that the output of the ampliiier may be applied directly across solenoid 21S in series with the resistor 218. This solenoid actuates what is known as a synchronizing relay consisting of two movable contacts 280 and 282, which are moved simultaneously by the actuation of the solenoid 275. The first of these, 28D, is connected through line 284 to the pulse generator 182, and the second is connected through line 286, which is not complete in order to simplyify it, to the electronic timer. A line 288 also eX- tends from the pulse generator to the electronic timer control 26. Switch contact 289 provides a circuit for the pulse generated by the pulse generator to trigger the electronic timer to initiate the slit adjustment period as desired at exactly the desired time and contact 28.2 upon deenergization of solenoid 218 provides a signal to the timer, indicating that the slit adjustment is completed and starting again the scanning process.

In the operation of the device. a continuous beam is projected from the source 2 through a sample 4 of any material which it desired to test or examine which then impinges upon the first slit 6. It is reduced by this slit to a predetermined desired value, and thenv impinges upon the prism 8, passing through the prism and being retracted into a spectrum, it falls upon the outlet slit lil, the amount passing therethrough impinging upon the pickup thermopile. As previously mentioned, the prism 8 is so mounted that it may be very slowly rotated about its vertical axis, so that the spectrum will very slowly scan over the slit t8, applying to the thermopi'le the various wave lengths cf tl e total spectrum uti,- liz-ed. This radiation falling upon the thermocouple pick-up generates energy therein. to actuate the recording apparatus lill! and: Hi8 previously described to produce a continuous record. Under these conditions prism. rotating motor H8, recorder motor H34 and chart driving motor HIJ are energized.

As before mentioned, it is essential to provide maximum resolution for the beams that proceed through the inlet and outlet slits, and,` of course, the energy in the beam is also dependent upon its wave length. Thus, the slit would have to be wider at one end of the spectrum to permit a sufficient amount of energy to pass through toprovide a satisfactory signal than it would be at some point further along the scanning position. As the amount of energy increases, the slits can `be reduced: in width, and, therefore, provide l etadjustability of these slits may be automatically carriedout in a series oi steps. Assuming that a portion of the spectrum is reachedwhich requires either a narrowing or widening of the slits 6 and I0, then it is desired to stop each of the above namedmotors, make the necessary slit adjustments, and then continue with the recording. In order to make the necessary slit adjustments, a sequence of operations is initiated by commutator |94 and controlled in order by the program electronic timer |26. The various steps in their sequence is as follows:

(1) Commutator |94 reaches a point at which it contacts a new segment to introduce a diierent control bridge circuit, such, for example, as 2|8. l

'(2) This energizes the amplier 250 which actuates contact 280 through coil 216, which sets the electronic timer in condition for initiation.

(3) rThe next pulse generated by the pulse generator |82 initiates the electronic timer to carry out its program sequence.

(4) Prism motor H8, recorder motor |94 and chart motor H are stopped by the electronic control (not shown in detail). y

, (5) VShutter i8 is next closed through energization of coil 22.

(6) Slit adjusting motor i6 is energized through energization of coil 268 and by its motion adjusts the size of the slits.

(7) The operation of slit motor |6 simultaneously moves contact 243 on resistor 238 in the bridge circuit until the latter is balanced to a value proportional to the setting on the potentiometer, such as 2|8. At the same time, of course, the slit width is varied..

(8) Coil 2'|6 now becomes deenergized due to the output of the amplifier 250 becoming zero and switches'280- and 282 open, which through the latter conditions the electronic timer for its next step.

`(9) Relay coil 06 is now energized to complete a circuit to zeroing motor 64 to balance the input signal to the amplifier 30 to zero. This operation also opens Contact S0 so that no voltage will be applied from potentiometer |28, and thirdly, switches arm |58 from the recorder dead spot control H4 to the zeroing dead spot control |68.

(10) Shutter i8 is now open as coil 22 becomes deenergized.

(1l) Recorder balancing motor |04 is now actuated and sufcient time is provided to bring the main circuit into balance. v

(l2) Prism motor |13 and chart motor` i0 are reenergized to again place the system in normal recording operation in which it will continue until it is necessary to again cause adjustment in slit width, due to the introduction of additional potentiometer means set at different values.

The pulse generator |82 also applies a pulse through line 200 to place a mark on the chart Hi8, identifying certain time intervals or degree marks for rotation of the prism.

`can switchto the next potentiometer.` In some instances it' may be desirable to shift or change the resolution of the Whole system for the complete program without changing each of the step settings individually. This is accomplished by a resolution control 222, which introduces series resistance'into the bridge circuit by changing its contact position.

Motor |04 is, of course, the recorder motor and is operated by the output of the amplifier 30, which picks up the signal from the thermopile. Thismotor moves the recording stylus or pen I 06 with regard to the chart, and its position is Vdetermined by the output of the amplier. The speed of response of the signal may be varied by moving' the contactor 82 to include different ones of the condensers '14 through 80 as the needs require. To initially set the gain, adjustable slide |36 is positioned along vresistor |34 until the approximate gain is acquired, and the contactor |40 is adjusted along resistor |42 to complete the setting.

For setting or indexing the device, test signals may be applied either before the recording is started or during the same Iby contactor 44 being placed on contacts including resistors 46, 48, 50 or 52, depending upon the strength of the desired test signal, which is for the purpose of adjusting the amplier and the remaining portions of the circuit if necessary.

The main recording motor |04, which operates the stylus |06, simultaneously rotates the generator |14, which applies a signal back to the incoming lines in inverse phase to prevent overshooting. That is, if the signal applied to the recorder amplifier increases rapidly, and the motor |04 speeds up, a voltage will be generated by the generator ||4 and applied back to the incoming lines of the recorder in an attempt to prevent a too-sudden rise in the same. This will, of course, minimize the possibility of overshooting. Upon rotation of motor |04, as Well as moving the recording pen and operating the anti-hunt generator the same moves the recorder contactor |28 across the slide wire |30 in an attempt to bring back to balance the bridge system to null, as is customary with this type of recorder.

Thus, as the varying degrees of energy fall upon the thermopile, the output thereof is amplied by ampliiier 30, which signal is applied to the recorder amplifier |02, whose motor |04 isenergized to operate the stylus |06 for tracing a mark upon the chart, provide a re-balance of the bridge system to null and generate an antiovershoot back voltage. Periodically, as the spectrum is scanned, the commutator |30 inserts a new potentiometer in the bridge controlling the slit adjustment, and this, being triggered by pulse generator |82, causes the motor |6 to be energized to periodically balance its bridge and adjust the width of the slits. At the end of the adjustment, the zeroing motor B4 adjusts its potentiometer to re-balance the system with the shutter closed, and the cycle is repeated.

I claim;

l. In spectrophotometric means, a source of radiation, a prism receiving rays therefrom and refracting the same into a spectrum, thermopile means to receive rays within the spectrum, opaque means having an adjustable opening therein interposed between the prism and the thermopile to meter the rays therethrough, means to rotate said prism so that the spectrum will scan the thermopile, driving means for adjusting the openings in the opaque means, balanced bridge means having a plurality of potentiometers in parallel in one side which are capable ci independent operation, switching means for inserting individual ones into said bridge circuit, said switching means being actuated by said means for rotating said prism, said balanced bridge means being connected to said means for adjusting the openings so that upon switching in a diiierent potentiometer causing bridge unbalance the driving means for adjusting the openings is energized, which last named driving means stops upon balancing of the bridge.

2. In spectrophotometric means having a thermopile and prism for refracting a spectrum of waves of energy thereon, opaque means between the prism and thermopile having an adjustable opening therein to control the rays impinging on the thermopile, driving means to turn the prism about its axis to cause the spectrum to scan over the opaque means and thermopile, means to adjust the size of the openings, bridge means controlling the opening adjusting means, alternate resistance means for forming one side of the bridge, and switch means to include one or the other of the resistance means in the bridge so that the openings may be adjusted by the insertion of preferred resistances.

3. In spectrcphotometric means having a thermopile and prism for reiracting a spectrum of Waves of energy thereon, opaque means between the prism and thermopile having an adjustable opening therein to control the rays impinging on the thermopile, driving means to turn the prism about its axis to cause the spectrum to scan over the opaque means and thermopile, means to adjust the size of the openings, a bridge formed of two potentiometers controlling the means to adjust the openings, a series of potentiometers in parallel with one of the rst named potentiometers capable of being set to diierent predetermined values of resistance and automatic switching means connected to and actuated -by the driving means for the prism to periodically introduce singly into circuit one of the series of potentiometers so that the value of the bridge will be changed according to the program setting on the potentiometers and thus periodically energize the controlling means to adjust the openings and they will follow a prescribed program as the prism is turned.

e. In spectrophotometric means having a thermopile and a prism lor reiracting a spectrum of waves oi energy, an adjustable slit for metering the energy railing on the thermopile between the prism and thermopile, a motor for turning the prism about its axis to cause the spectrum to scan the slit, a second motor to adjust the size oi the slit, a balanced bridge circuit for controlling the second motor, a plurality of potentiometers in parallel connectable with one side oi the bridge, commutator switching means connected to the shaft of the iirst motor to periodically switch the potentionieters in sequence into the bridge circuit and cause varying degrees of unbalance to cause the second motor to run until rebalance-d and thus change the slit size depend-ent upon the settings on the potentiometers.

5. In spectrophotometric means having a thermopile and a prism for refracting a spectrum of waves oi energy, an adjustable slit for metering the energy falling on the thermopile between the prism and thermopile, a motor for turning the prism about its axis to cause the spectrum to scan the slit, a second motor to adjust the size of the slit, a balanced bridge circuit for controlling the second motor, a plurality of potentiom- '10 eters in .parallel connectable with one side the bridge, commutator switching means connected to the shaft of the first motor to periodically switch the potentiometers in sequence into the bridge circuit and cause varying degrees of unbalance to cause the second motor to run until rebalanced and thus change the 'slit size dependent upon the settings on the potentiometers, electronic timing means, pulse generating means also driven by the first motor to trigger the energization of the second motor.

6. In spectrophotometric means having a thermopile and a prism for refracting a spectrum or waves of energy, an adjustable slit for metering the-energy falling on the thermopile vbetween the prism and thermopile, a motor for tuning the prism about its axis to cause the spectrum to scan the slit, a second motor to adjust the size of the slit, a balanced bridge circuit for controlling the second motor, a plurality of potentiometers in parallel connectable with one side of the bridge, commutator switching means connected to the shaft of the first motor to periodically switch the potentiometers in sequence into the bridge circuit and cause varying degrees of unbalance to cause the second motor to run until rebalanced and thus-change the slit size dependentupon the settings on the .potentiometers, and a plurality oi tapped resistances in seriesbetween the parallel potentiometers and one end of the bridge to change the whole balance of the 'bridge in steps which remain in circuit for the full run.

7. In spectrophotometric means having a thermopile and prism for refracting a beam of energy into a spectrum, an adjustable slit -or varying the amount of spectrum incident upon the thermopile, a iirst motor to turn the prism about its axis to scan the slit, a second motor to adjust the slit width, a control bridge for the second motor, an amplier between the bridge and motor, a commutator switch driven by the first motor to sequentially change the balance of the bridge as the prism rotates to change the slit width according to a preset program.

v8. In spectrophotometric means having a thermopile and prism for refracting a beam of energy into/a spectrum, an adjustable slit for varying the amount of spectrum incident upon the thermopile, a rst motor to turn the prism about its axis to scan the slit, a second motor to adjust the slit width, a control bridge for the second motor, an amplifier between the bridge and motor, a commutator switch driven by the rst motor to sequentially change the balance of the bridge as the prism rotates to change the slit width according to a preset program, electronic timing means, switch means between the amplier and the second motor controlled by the timing means to apply the correcting adjustment at the desired moment.

9. In spectrophotometric means having a thermopile and prism for reiracting a beam of energy into a spectrum, an adjustable slit for varying the amount of spectrum incident upon the thermopile, a first motor to turn the prism about its axis to scan the slit, a second motor to adjust the slit width, a control bridge for the second motor, an amplifier between the bridge and motor, a commutator switch driven by the iirst motor to sequentially change the balance of the bridge as the prism rotates to change the slit width according to a preset program, electronic timing means, switch means between the amplier and the second motor controlled by the timing means to apply the correcting adjustment at 111 the desired moment, means to manually balance the bridge, and switching means to permit either manual or automatic bridge adjustment to ccntrol the slits.

10. In spectrophotometric means having a thermopile and prism for refracting a beam of energy into a spectrum, an adjustable slit for varying the amount of spectrum incident upon the thermopile, a rst motor to turn the prism about its axis to scan the slit, a second motor to adjust the slit width, an amplifier connected to the thermopile, balancing circuit means between the thermopile and amplifier, a control bridge for the second motor, a commutator switch driven by the rst motor to sequentially change the balance of the bridge as the prism is rotated to vary the slit width according to a program, shutter means to cut off the energyv ilow to the prism during slit adjustment and means to bring to zero the balancing circuit in the thermopile output.

11. In spectrophotometric means .having a thermopile and prism for refracting a beam of energy into a spectrum, an adjustable slit for varying the amount of spectrum incident upon the thermopile, a rst motor to turn the prism about its axis to scan the slit, a second motor to adjust the slit Width, an amplier connected to the thermopile, balancing circuit means between the thermopile and amplier, a control bridge for the second motor, a commutator switch driven by the rst motor to sequentially change the balance of the bridge as the prism is rotated to vary the slit width according to a program,

shutter means to cut oi the energy ow to the Y prism during slit adjustment, means to bring to zero the balancing circuit in the thermopile output, and means to apply test signals of diierent strengths to the amplier input at any time,

12. In spectrophotometric means, a source of radiation, a prism receiving rays therefrom `and refracting the same into a spectrum, thermopile means to receive rays within the spectrum, opaque means having an adjustable opening therein interposed between the prism and the thermopile to meter the rays therethrough, v.means to cause relative motion between the prism and the thermopile so that the spectrum will scan the there mopile, driving means for adjusting the size of the opening in the opaque means, balanced bridge means connected to the driving means, variable resistance means forming one arm of the bridge and being connected to and actuated by the means for causing relative motion between the prism and thermopile so that as the spectrum scans the thermopile the bridge balance will be changed and cause the opening size to Vary.

13. In spectrophotometric means, having a thermopile and prism for refracting a spectrum of waves thereon, opaque means interposed between the thermopile and prism having an adjustable aperture therein to control the amount of energy passing to the thermopile, means for causing relative movement between the prism and the thermopile so that a spectrum of wave lengths will scan the thermopile, driving means to adjust the size of the aperture in the opaque means, balanced 'bridge means connected to the driving means, one arm of said bridge being formed of a plurality of potentiometers in parallel, switching means to connect one of the potentiometers into the bridge circuit at a time to change the bridge balance, depending upon the setting of the potentiometer included, and means for operating said switching means connected to and driven by the means for causing relative movement between the prism and the thermopile.

MAX D. LISTON.

REFERENCES CrrED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number V Name Date 1,894,132 Stone, Jan, 10, 1933 2,063,140 Allison Dec. 8, 1936 2,170,435 Sweeney Aug. 22, 1939 2,176,013 Pineo Oct. 19, 1939 2,218,357 Pineo Oct. 15, 1940 2,314,800 Pineo Mar. 23, 1943 2,404,064 Heigl et al July 16, 1946 

